Processes for desulfurizing coal are well known in the art. Most of these processes are useful for reducing the pyritic content of coal, but have little or no effect on reducing the organic or elemental sulfur in coal.
One process is set forth in U.S. Pat. No. 3,960,513 which discloses a process for reducing the amount of sulfur in coal by subjecting an aqueous slurry of coal to pressure oxygen leaching. In one embodiment, the aqueous solution may be acidic. However, this treatment removes mainly pyritic sulfur. Removal of organic sulfur requires an additional step of treating the coal with oxygen and a solvent such as ammonium hydroxide. Further, removal of the elemental sulfur requires a separate extraction with a solvent such as kerosene or toluene.
Removal of pyritic sulfur by means of an acid leach solution or an oxygen-containing gas is also disclosed in U.S. Pat. No. 4,083,696.
Another approach to removal of pyritic sulfur from coal is set forth in U.S. Pat. Nos. 3,926,575 and 3,768,988. This process comprises reacting finely divided coal with sulfuric acid and, optionally, hydrochloric acid to form ferric chloride which presumably reacts with the pyrite in the coal to generate free sulfur. In addition to an acid leach, a final solvent extraction step may be included to remove further free sulfur in the coal. If the solvent is para cresol, apparently a portion of organic sulfur compounds contained in the coal can also be removed.
U.S. Pat. No. 4,305,726 relates to a method for removing pyritic sulfur and ash from coal. The method involves treating coal with spent pickle liquor and reacting in the presence of hydrochloric acid, hypochorous acid and oxygen gas formed by adding the coal/pickle liquor mixture to a reactor with water and chlorine gas. After reaction, the ash is removed by mechanical means, for example by use of a weir.
In addition to the above-described desulfurization methods, methods are known which employ metal chloride salts. For example, U.S. Pat. No. 3,909,213 discloses a process for desulfurization of coal which comprises digesting coal with a Group IA or IIA metal oxide and a fused metal chloride salt such as zinc or ferric chloride. The medium is apparently capable of dissolving sulfur-containing organic compounds present in the coal. Anhydrous hydrogen chloride is passed through the digestion zone. U.S. Pat. No. 4,127,390 discloses a process employing an aqueous solution of sodium chloride for extracting pyritic sulfur from coal.
In addition to the numerous examples of processes set forth above, it is also known that organic sulfates can be extracted from coal by the use of solvents such as combustible sulfur solvents (U.S. Pat. No. 4,203,727) and chlorinated organic solvents (U.S. Pat. No. 4,081,250).
Despite the numerous above-described processes for desulfurizing coal, it is apparent that there still remains room for improvement. In particular, a more efficient, more economical method of desulfurizing coal and other carbonaceous materials would be desirable.